High Discharge Capacity Lithium Battery

ABSTRACT

A lithium/iron disulfide electrochemical battery cell with a high discharge capacity. The cell has a lithium negative electrode, an iron disulfide positive electrode and a nonaqueous electrolyte. The positive electrode mixture containing the iron disulfide contains highly packed solid materials, with little space around the solid particles, to provide a high concentration of iron disulfide within the mixture. The separator is thin, to allow more space within the cell for active materials, yet strong enough to prevent short circuits between the positive and negative electrodes under abusive conditions, even when swelling of the cathode during cell discharge places additional stressed on the separator. As a result, the ratio of the interfacial capacity of the positive electrode to the electrode interfacial volume is high, as is the actual capacity on low rate/low power and high rate/high power discharge.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/779,555, filed on May 13, 2010 which is a continuation of pendingUnited States Patent Publication No. 2008-0076022(A1), Ser. No.11/949,356 filed on Dec. 3, 2007, which was a continuation of UnitedStates Patent Publication No. 2005-0112462(A1), Ser. No. 10/719,425filed on Nov. 21, 2003 (now abandoned). This application also claimspriority to pending U.S. patent application Ser. No. 12/706,968 filed onFeb. 17, 2010, which is a continuation-in-part of United States PatentPublication 2005-0233214(A1), Ser. No. 11/020,339 filed on Dec. 22, 2004(now abandoned), which is itself a continuation in part of theaforementioned United States Patent Publication No. 2005-0112462(A1),Ser. No. 10/719,425 filed on Nov. 21, 2003 (now abandoned).

BACKGROUND

This invention relates to electrochemical battery cells, particularlycells with a lithium negative electrode and an iron disulfide positiveelectrode.

Lithium batteries (batteries containing metallic lithium as the negativeelectrode active material) are becoming increasingly popular as portablepower sources for electronic devices that have high power operatingrequirements. Common consumer lithium batteries includelithium/manganese dioxide (Li/MnO₂) and lithium/iron disulfide (Li/FeS₂)batteries, which have nominal voltages of 3.0 and 1.5 volts per cell,respectively.

Battery manufacturers are continually striving to design batteries withmore discharge capacity. This can be accomplished by minimizing thevolume in the cell taken up by the housing, including the seal and thevent, thereby maximizing the internal volume available for activematerials. However, there will always be practical limitations on themaximum internal volume.

Another approach is to modify the internal cell design and materials toincrease the discharge capacity. How to best accomplish this can dependat least in part on the discharge requirements of the devices to bepowered by the batteries. For devices with low power requirements, thequantity of active materials tends to be very important, while fordevices with high power requirements, discharge efficiencies tend to bemore important. Lithium batteries are often used in high power devices,since they are capable of excellent discharge efficiencies on high powerdischarge.

In general, battery discharge efficiency decreases rapidly withincreasing discharge power. Therefore, for high power providing highdischarge efficiency is a priority. This often means using designscontaining less active materials, thus sacrificing capacity on low powerand low rate discharge. For example, for good high power dischargeefficiency, high interfacial surface area between the negative electrode(anode) and positive electrode (cathode) relative to the volume of theelectrodes is desirable. This is often accomplished by using a spirallywound electrode assembly, in which relatively long, thin electrodestrips are wound together in a coil. Unless the electrode compositionshave a high electrical conductivity, such long, thin electrodestypically require a current collector extending along much of the lengthand width of the electrode strip. The high interfacial surface area ofthe electrodes also means that more separator material is needed toelectrically insulate the positive and negative electrodes from eachother. Because the maximum external dimensions are often set for thecells, either by industry standards or the size and shape of the batterycompartments in equipment, increasing the electrode interfacial surfacearea also means having to reduce the amount of active electrodematerials that can be used.

For batteries that are intended for both high and low power use,reducing cell active material inputs in order to maximize high powerperformance is less desirable than for batteries intended for only highpower use. For example, AA size 1.5 volt Li/FeS₂ (FR6 size) batteriesare intended for use in high power applications such as photoflash anddigital still camera as well as general replacements for AA size 1.5volt alkaline Zn/MnO₂ batteries, which are often used in lower powerdevices. In such situations it is important to maximize both high powerdischarge efficiency and cell input capacity. While it is generallydesirable to maximize the electrode input capacity in any cell, therelative importance of doing so is greater in cells for lower powerusage.

To maximize the active material inputs in the cell and mitigate theeffects thereon of increasing the electrode interfacial surface area, itis desirable to use separator materials that take up as little internalvolume in the cell as possible. There are practical limitations to doingso. The separator must be able to withstand the cell manufacturingprocesses without damage, provide adequate electrical insulation and iontransport between the anode and cathode and do so without developingdefects resulting in internal short circuits between the anode andcathode when the cell is subjected to both normal and anticipatedabnormal conditions of handling, transportation, storage and use.

Separator properties can be modified in a number of ways to improve thestrength and resistance to damage. Examples are disclosed in U.S. Pat.Nos. 5,952,120; 6,368,742; 5,667,911 and 6,602,593. However, changesmade to increase strength can also adversely affect separatorperformance, based in part on factors such as cell chemistry, electrodedesign and features, cell manufacturing process, intended cell use,anticipated storage and use conditions, etc.

For certain cell chemistries maximizing the amounts of active materialsin the cell can be more difficult. In lithium batteries, when the activecathode material reacts with the lithium to produce reaction productshaving a total volume greater than that of the reactants, swelling ofthe electrode assembly creates additional forces in the cell. Theseforces can cause bulging of the cell housing and short circuits throughthe separator. Possible solutions to these problems include using strong(often thicker) materials for the cell housing and inert componentswithin the cell, further limiting the internal volume available foractive materials in cells with such active materials compared to cellswith lower volume reaction products. For Li/FeS₂ cells another possiblesolution, disclosed in U.S. Pat. No. 4,379,815, is to balance cathodeexpansion and anode contraction by mixing another active material withthe FeS₂. Such active cathode materials include CuO, Bi₂O₃, Pb₂Bi₂O₅,P₃O₄, CoS₂ and mixtures thereof. However, adding other active materialsto the cathode mixture can affect the electrical and dischargecharacteristics of the cell.

Just as battery manufacturers are continually trying to improvedischarge capacity, they are also continually working to improve otherbattery characteristics, such as safety and reliability; making cellsmore resistant to internal short circuits can contribute to both. As isclear from the above discussion, changes made to improve resistance tointernal short circuits can be counterproductive in maximizing dischargecapacity.

In view of the above, an object of the present invention is to provide alithium battery cell with increased discharge capacity. Another objectof the invention is to provide a lithium battery cell with a high energydensity (interfacial discharge capacity to interfacial electrodevolume). Another object of the invention is to provide a Li/FeS₂ cellwith a high interfacial electrode surface area that has increaseddischarge capacity on low power discharge without sacrificing dischargeefficiency on high power discharge, preferably one with increaseddischarge capacity on both high rate and low rate discharge. Yet anotherobject of the invention is to provide a Li/FeS₂ cell with increasedcathode interfacial capacity and having both improved energy density andgood resistance to internal short circuits.

SUMMARY

The above objects are met and the above disadvantages of the prior artare overcome by the present invention.

Accordingly, one aspect of the present invention is directed to anelectrochemical battery cell comprising a housing; a negative electrodestrip comprising metallic lithium, a positive electrode strip comprisingan active material mixture and an electrolyte comprising at least onesalt dissolved in a nonaqueous electrolyte disposed within the housing;and a separator disposed between the negative and positive electrodes;the cell having a ratio of a cathode interfacial capacity to anelectrode assembly interfacial volume of at least 710 mAh/cm³.

Another aspect of the present invention is directed to anelectrochemical battery cell comprising a housing; a negative electrode,a positive electrode and an electrolyte disposed within the housing; anda separator disposed between the negative and positive electrodes. Thehousing comprises a cylindrical container with an integral closed bottomend, an initially open top end, a side wall extending between the bottomand top ends and a cover disposed in the top end to close the cell; thenegative electrode is in the form of a strip with two opposing majorsurfaces and comprises metallic lithium; the positive electrode is inthe form of a strip with two opposing major surfaces and comprises anactive material mixture, the active material comprising greater than 50weight percent iron disulfide; the electrolyte comprises one or moresalts dissolved in a nonaqueous organic solvent; the negative andpositive electrodes and the separator form a spiral wound cylindricalelectrode assembly, with a radial outer surface disposed adjacent aninner surface of the container side wall; the electrode assembly has aninterfacial volume; the positive electrode has an interfacial capacity;a ratio of the positive electrode interfacial capacity to the electrodeassembly interfacial volume is at least 710 mAh/cm³; and the separatoris a microporous membrane comprising polyethylene, with a machinedirection and a transverse direction, an average thickness less than 22μm and a tensile stress of at least 1.0 kgf/cm in both the machinedirection and the transverse direction.

Another aspect of the present invention is directed to anelectrochemical battery cell comprising a housing; a negative electrode,a positive electrode and an electrolyte disposed within the housing; anda separator disposed between the negative and positive electrodes. Thecell is a cylindrical FR6 type Li/FeS₂ cell with a spiral woundelectrode assembly having an electrode assembly interfacial volume; thecell has an interfacial capacity of at least 3500 mAh; the separator isa microporous membrane comprising polyethylene and has an averagethickness less than 22 μm, a tensile stress of at least 2.0 kgf/cm inboth a machine direction and a transverse direction, a dielectricbreakdown voltage of at least 2400 volts, a maximum effective pore sizeof 0.08 μm to 0.20 μm and a BET specific surface area of 4.0 to 15 m²/g.

Yet another aspect of the present invention is directed to anelectrochemical battery cell comprising a housing; a negative electrode,a positive electrode and an electrolyte disposed within the housing; anda separator disposed between the negative and positive electrodes. Thecell is a cylindrical FR6 type Li/FeS₂ cell with a spiral woundelectrode assembly having an electrode assembly interfacial volume; theseparator is a microporous membrane comprising polyethylene and has anaverage thickness less than 22 μm, a tensile stress of at least 2.0 inboth a machine direction and a transverse direction, a dielectricbreakdown voltage of at least 2400 volts and a maximum effective poresize of 0.08 μm to 0.20 μm; the positive electrode comprises an activematerial comprising at least 95 weight percent iron disulfide; and thecell is capable of providing a discharge capacity of at least 2950 mAhwhen discharged at 200 mA continuously to 1.0 volt and a dischargecapacity of at least 2600 mAh when discharged at 1000 mA continuously to1.0 volt.

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

Unless otherwise specified, as used herein the terms listed below aredefined as follows:

-   -   active material—one or more chemical compounds that are part of        the discharge reaction of a cell and contribute to the cell        discharge capacity, including impurities and small amounts of        other moieties present;    -   active material mixture—a mixture of solid electrode materials,        excluding current collectors and electrode leads, that contains        the electrode active material;    -   capacity, discharge—the actual capacity delivered by a cell        during discharge, generally expressed in amp-hour (Ah) or        milliamp-hours (mAh);    -   capacity, input—the theoretical capacity of an electrode, equal        to the weight of each active material in the electrode times the        theoretical specific capacity of that active material, where the        theoretical specific capacity of each active material is        determined according to the following calculation:

[(96,487 ampere-seconds/mole)/(number of grams/mole of activematerial)]×(number of electrons/mole of active material)/(3600seconds/hour)×(1000 milliampere hours/ampere-hour)

-   -   (e.g., Li=3862.0 mAh/g, S=1672.0 mAh/g, FeS₂=893.6 mAh/g,        CoS₂—871.3 mAh/g, CF_(x)=864.3 mAh/g, CuO=673.8 mAh/g, C₂F=623.0        mAh/g, FeS=609.8 mAh/g, CuS=560.7 mAh/g, Bi₂O₃=345.1 mAh/g,        MnO₂=308.3 mAh/g, Pb₂Bi₂O₅=293.8 mAh/g and FeCuS₂—292.1 mAh/g);    -   capacity, cell interfacial—the smaller of the negative and        positive electrode capacity;    -   capacity, electrode interfacial—the total contribution of an        electrode to the cell theoretical discharge capacity, based on        the overall cell discharge reaction mechanism(s) and the total        amount of active material contained within the that portion of        the active material mixture adjacent to active material in the        opposite electrode, assuming complete reaction of all of the        active material, generally expressed in Ah or mAh (where only        one of the two major surfaces of an electrode strip is adjacent        active material in the opposite electrode, only the active        material on that side of the electrode—either the material on        that side of a solid current collector sheet or that material in        half the thickness of an electrode without a solid current        collector sheet—is included in the determination of interfacial        capacity);    -   electrode assembly—the combination of the negative electrode,        positive electrode, and separator, as well as any insulating        materials, overwraps, tapes, etc., that are incorporated        therewith, but excluding any separate electrical lead affixed to        the active material, active material mixture or current        collector;    -   electrode loading—active material mixture dry weight per unit of        electrode surface area, generally expressed in grams per square        centimeter (g/cm²);    -   electrode packing—active material dry weight per unit of        electrode surface area divided by the theoretical active        material mixture dry weight per unit of electrode surface area,        based on the real densities of the solid materials in the        mixture, generally expressed as a percentage;    -   folded electrodes—electrode strips that are combined into an        assembly by folding, with the lengths of the strips either        parallel to or crossing one another;    -   interfacial height, electrode assembly—the average height,        parallel to the longitudinal axis of the cell, of the        interfacial surface of the electrodes in the assembly;    -   interfacial volume, electrode assembly—the volume within the        cell housing defined by the cross-sectional area, perpendicular        to the longitudinal axis of the cell, at the inner surface of        the container side wall(s) and the electrode assembly        interfacial height;    -   nominal—a value, specified by the manufacturer, that is        representative of what can be expected for that characteristic        or property;    -   percent discharge—the percentage of the rated capacity removed        from a cell during discharge;    -   spiral wound electrodes—electrode strips that are combined into        an assembly by winding along their lengths or widths, e.g.,        around a mandrel or central core; and    -   void volume, electrode assembly—the volume of the electrode        assembly voids per unit of interfacial height, determined by        subtracting the sum of the volumes of the non-porous electrode        assembly components and the solid portions of the porous        electrode assembly components contained within the interfacial        height from the electrode assembly interfacial volume        (microporous separators, insulating films, tapes, etc. are        assumed to be non-porous and non-compressible, and volume of a        porous electrode is determined using the real densities of the        components and the total actual volume), generally expressed in        cm³/cm.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an embodiment of the electrochemical battery cell of theinvention; and

FIG. 2 is a graph showing Impact Test results for partially dischargedFR6 cells as a function of the volume of voids per unit height of theelectrode assembly within the interfacial height.

DESCRIPTION

The battery cell of the invention has an anode comprising metalliclithium as the negative electrode active material. The anode and cathodeare both in the form of strips, which are joined together in anelectrode assembly to provide a high interfacial surface area relativeto the volumes of the electrodes containing active material. The higherthe interfacial surface area, the lower the current density and thebetter the cell's capability to deliver high power on discharge. Thecell also has a high ratio of cathode interfacial capacity to electrodeassembly interfacial volume—at least 710 mAh/cm². This means that thevolume of active materials in the electrode assembly is high, to providea high discharge capacity. The high volume of active materials can beachieved by controlling a number of variables, including: the ratio ofinterfacial input capacity to total input capacity, the volume of thecathode current collector, the concentration of active cathode materialin the cathode mixture and the volume of separator in the electrodeassembly.

The invention will be better understood with reference to FIG. 1, whichshows an embodiment of a cell according to the invention. Cell 10 is anFR6 type cylindrical Li/FeS₂ battery cell. Cell 10 has a housing thatincludes a can 12 with a closed bottom and an open top end that isclosed with a cell cover 14 and a gasket 16. The can 12 has a bead orreduced diameter step near the top end to support the gasket 16 andcover 14. The gasket 16 is compressed between the can 12 and the cover14 to seal an anode 18, a cathode 20 and electrolyte within the cell 10.The anode 18, cathode 20 and a separator 26 are spirally wound togetherinto an electrode assembly. The cathode 20 has a metal current collector22, which extends from the top end of the electrode assembly and isconnected to the inner surface of the cover 14 with a contact spring 24.The anode 18 is electrically connected to the inner surface of the can12 by a metal tab (not shown). An insulating cone 46 is located aroundthe peripheral portion of the top of the electrode assembly to preventthe cathode current collector 22 from making contact with the can 12,and contact between the bottom edge of the cathode 20 and the bottom ofthe can 12 is prevented by the inward-folded extension of the separator26 and an electrically insulating bottom disc 44 positioned in thebottom of the can 12. Cell 10 has a separate positive terminal cover 40,which is held in place by the inwardly crimped top edge of the can 12and the gasket 16. The can 12 serves as the negative contact terminal.Disposed between the peripheral flange of the terminal cover 40 and thecell cover 14 is a positive temperature coefficient (PTC) device 42 thatsubstantially limits the flow of current under abusive electricalconditions. Cell 10 also includes a pressure relief vent. The cell cover14 has an aperture comprising an inward projecting central vent well 28with a vent hole 30 in the bottom of the well 28. The aperture is sealedby a vent ball 32 and a thin-walled thermoplastic bushing 34, which iscompressed between the vertical wall of the vent well 28 and theperiphery of the vent ball 32. When the cell internal pressure exceeds apredetermined level, the vent ball 32, or both the ball 32 and bushing34, is forced out of the aperture to release pressurized gases from thecell 10.

The cell container is often a metal can with an integral closed bottom,though a metal tube that is initially open at both ends may also be usedinstead of a can. The can is generally steel, plated with nickel on atleast the outside to protect the outside of the can from corrosion. Thetype of plating can be varied to provide varying degrees of corrosionresistance or to provide the desired appearance. The type of steel willdepend in part on the manner in which the container is formed. For drawncans the steel can be a diffusion annealed, low carbon, aluminum killed,SAE 1006 or equivalent steel, with a grain size of ASTM 9 to 11 andequiaxed to slightly elongated grain shape. Other steels, such asstainless steels, can be used to meet special needs. For example, whenthe can is in electrical contact with the cathode, a stainless steel maybe used for improved resistance to corrosion by the cathode andelectrolyte.

The cell cover is typically metal. Nickel plated steel may be used, buta stainless steel is often desirable, especially when the cover is inelectrical contact with the cathode. The complexity of the cover shapewill also be a factor in material selection. The cell cover may have asimple shape, such as a thick, flat disk, or it may have a more complexshape, such as the cover shown in FIG. 1. When the cover has a complexshape like that in FIG. 1, a type 304 soft annealed stainless steel withASTM 8-9 grain size may be used, to provide the desired corrosionresistance and ease of metal forming. Formed covers may also be plated,with nickel for example.

The terminal cover should have good resistance to corrosion by water inthe ambient environment, good electrical conductivity and, when visibleon consumer batteries, an attractive appearance. Terminal covers areoften made from nickel plated cold rolled steel or steel that is nickelplated after the covers are formed. Where terminals are located overpressure relief vents, the terminal covers generally have one or moreholes to facilitate cell venting.

The gasket is made from any suitable thermoplastic material thatprovides the desired sealing properties. Material selection is based inpart on the electrolyte composition. Examples of suitable materialsinclude polypropylene, polyphenylene sulfide,tetrafluoride-perfluoroalkyl vinylether copolymer, polybutyleneterephthalate and combinations thereof. Preferred gasket materialsinclude polypropylene (e.g., PRO-FAX® 6524 from Basell Polyolefins,Wilmington, Del., USA), polybutylene terephthalate (e.g., CELANEX® PBT,grade 1600A from Ticona-US, Summit, N.J., USA) and polyphenylene sulfide(e.g., TECHTRON® PPS from Boedeker Plastics, Inc., Shiner, Tex., USA).Small amounts of other polymers, reinforcing inorganic fillers and/ororganic compounds may also be added to the base resin of the gasket.

The gasket may be coated with a sealant to provide the best seal.Ethylene propylene diene terpolymer (EPDM) is a suitable sealantmaterial, but other suitable materials can be used.

The vent bushing is made from a thermoplastic material that is resistantto cold flow at high temperatures (e.g., 75° C.). The thermoplasticmaterial comprises a base resin such as ethylene-tetrafluoroethylene,polybutylene terephthlate, polyphenylene sulfide, polyphthalamide,ethylene-chlorotrifluoroethylene, chlorotrifluoroethylene,perfluoroalkoxyalkane, fluorinated perfluoroethylene polypropylene andpolyetherether ketone. Ethylene-tetrafluoroethylene copolymer (ETFE),polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) andpolyphthalamide are preferred. The resin can be modified by adding athermal-stabilizing filler to provide a vent bushing with the desiredsealing and venting characteristics at high temperatures. The bushingcan be injection molded from the thermoplastic material. TEFZEL® HT2004(ETFE resin with 25 weight percent chopped glass filler) is a preferredthermoplastic material.

The vent ball can be made from any suitable material that is stable incontact with the cell contents and provides the desired cell sealing andventing characteristic. Glasses or metals, such as stainless steel, canbe used.

The anode comprises a strip of lithium metal, sometimes referred to aslithium foil. The composition of the lithium can vary, though forbattery grade lithium the purity is always high. The lithium can bealloyed with other metals, such as aluminum, to provide the desired cellelectrical performance. Battery grade lithium-aluminum foil containing0.5 weight percent aluminum is available from Chemetall Foote Corp.,Kings Mountain, N.C., USA.

The anode may have a current collector, within or on the surface of themetallic lithium. As in the cell in FIG. 1, a separate current collectormay not be needed, since lithium has a high electrical conductivity, buta current collector may be included, e.g., to maintain electricalcontinuity within the anode during discharge, as the lithium isconsumed. When the anode includes a current collector, it may be made ofcopper because of its conductivity, but other conductive metals can beused as long as they are stable inside the cell.

A thin metal strip often serves as an electrical lead, or tab,connecting the anode to one of the cell terminals (the can in the caseof the FR6 cell shown in FIG. 1). The metal strip is often made fromnickel or nickel plated steel and affixed directly to the lithium. Thismay be accomplished embedding an end of the lead within a portion of theanode or by simply pressing an end of the lead onto the surface of thelithium foil.

The cathode is in the form of a strip that comprises a current collectorand a mixture that includes one or more electrochemically activematerials, usually in particulate form. Iron disulfide (FeS₂) is apreferred active material. In a Li/FeS₂ cell the active materialcomprises greater than 50 weight percent FeS₂. The cathode can alsocontain one or more additional active materials, depending on thedesired cell electrical and discharge characteristics. The additionalactive cathode material may be any suitable active cathode material.Examples include Bi₂O₃, C₂F, CF_(x), (CF)_(n), CoS₂, CuO, CuS, FeS,FeCuS₂, MnO₂, Pb₂Bi₂O₅ and S. More preferably the active material for aLi/FeS₂ cell cathode comprises at least 95 weight percent FeS₂, yet morepreferably at least 99 weight percent FeS₂, and most preferably FeS₂ isthe sole active cathode material. Battery grade FeS₂ having a puritylevel of at least 95 weight percent is available from American Minerals,Inc., Camden, N.J., USA; Chemetall GmbH, Vienna, Austria; and KyaniteMining Corp., Dillwyn, Va., USA.

In addition to the active material, the cathode mixture contains othermaterials. A binder is generally used to hold the particulate materialstogether and adhere the mixture to the current collector. One or moreconductive materials such as metal, graphite and carbon black powdersmay be added to provide improved electrical conductivity to the mixture.The amount of conductive material used can be dependent upon factorssuch as the electrical conductivity of the active material and binder,the thickness of the mixture on the current collector and the currentcollector design. Small amounts of various additives may also be used toenhance cathode manufacturing and cell performance. The following areexamples of active material mixture materials for Li/FeS₂ cell cathodes.Graphite: KS-6 and TIMREX® MX15 grades synthetic graphite from TimcalAmerica, Westlake, Ohio, USA. Carbon black: Grade C55 acetylene blackfrom Chevron Phillips Company LP, Houston, Tex., USA. Binder:ethylene/propylene copolymer (PEPP) made by Polymont Plastics Corp.(formerly Polysar, Inc.) and available from Harwick StandardDistribution Corp., Akron, Ohio, USA; non-ionic water solublepolyethylene oxide (PEO): POLYOX® from Dow Chemical Company, Midland,Mich., USA; and G1651 grade styrene-ethylene/butylenes-styrene (SEBS)block copolymer from Kraton Polymers, Houston, Tex. Additives: FLUO HT®micronized polytetrafluoroethylene (PTFE) manufactured by Micro PowdersInc., Tarrytown, N.Y., USA (commercially available from Dar-Tech Inc.,Cleveland, Ohio, USA) and AEROSIL® 200 grade fumed silica from DegussaCorporation Pigment Group, Ridgefield, N.J.

The current collector may be disposed within or imbedded into thecathode surface, or the cathode mixture may be coated onto one or bothsides of a thin metal strip. Aluminum is a commonly used material. Thecurrent collector may extend beyond the portion of the cathodecontaining the cathode mixture. This extending portion of the currentcollector can provide a convenient area for making contact with theelectrical lead connected to the positive terminal. It is desirable tokeep the volume of the extending portion of the current collector to aminimum to make as much of the internal volume of the cell available foractive materials and electrolyte.

A preferred method of making FeS₂ cathodes is to roll coat a slurry ofactive material mixture materials in a highly volatile organic solvent(e.g., trichloroethylene) onto both sides of a sheet of aluminum foil,dry the coating to remove the solvent, calender the coated foil tocompact the coating, slit the coated foil to the desired width and cutstrips of the slit cathode material to the desired length. It isdesirable to use cathode materials with small particle sizes to minimizethe risk of puncturing the separator. For example, FeS₂ is preferablysieved through a 230 mesh (62 μm) screen before use.

The cathode is electrically connected to the positive terminal of thecell. This may be accomplished with an electrical lead, often in theform of a thin metal strip or a spring, as shown in FIG. 1. The lead isoften made from nickel plated stainless steel.

The separator is a thin microporous membrane that is ion-permeable andelectrically nonconductive. It is capable of holding at least someelectrolyte within the pores of the separator. The separator is disposedbetween adjacent surfaces of the anode and cathode to electricallyinsulate the electrodes from each other. Portions of the separator mayalso insulate other components in electrical contact with the cellterminals to prevent internal short circuits. Edges of the separatoroften extend beyond the edges of at least one electrode to insure thatthe anode and cathode do not make electrical contact even if they arenot perfectly aligned with each other. However, it is desirable tominimize the amount of separator extending beyond the electrodes.

To provide good high power discharge performance it is desirable thatthe separator have the characteristics (pores with a smallest dimensionof at least 0.005 μm and a largest dimension of no more than 5 μMacross, a porosity in the range of 30 to 70 percent, an area specificresistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5)disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and herebyincorporated by reference. Suitable separator materials should also bestrong enough to withstand cell manufacturing processes as well aspressure that may be exerted on the separator during cell dischargewithout tears, splits, holes or other gaps developing that could resultin an internal short circuit.

To minimize the total separator volume in the cell, the separator shouldbe as thin as possible, preferably less than 25 μm thick, and morepreferably no more than 22 μm thick, such as 20 μm or 16 Separators asthin as 10 μm or less can be used if they have suitable properties. Therequired thickness will depend in part on the strength of the separatormaterial and the magnitude and location of forces that may be exerted onthe separator where it provides electrical insulation.

A number of characteristics besides thickness can affect separatorstrength. One of these is tensile stress. A high tensile stress isdesirable, preferably at least 800, more preferably at least 1000kilograms of force per square centimeter (kgf/cm²). Because of themanufacturing processes typically used to make microporous separators,tensile stress is typically greater in the machine direction (MD) thanin the transverse direction (TD). The minimum tensile stress requiredcan depend in part on the diameter of the cell. For example, for a FR6type cell the preferred tensile stress is at least 1500 kgf/cm² in themachine direction and at least 1200 kgf/cm² in the transverse direction,and for a FR03 type cell the preferred tensile strengths in the machineand transverse directions are 1300 and 1000 kgf/cm², respectively. Ifthe tensile stress is too low, manufacturing and internal cell forcescan cause tears or other holes. In general, the higher the tensilestress the better from the standpoint of strength. However, if thetensile stress is too high, other desirable properties of the separatormay be adversely affected.

Tensile stress can also be expressed in kgf/cm, which can be calculatedfrom tensile stress in kgf/cm² by multiplying the later by the separatorthickness in cm. Tensile stress in kgf/cm is also useful for identifyingdesirable properties related to separator strength. Therefore, it isdesirable that the separator have a tensile stress of at least 1.0kgf/cm, preferably at least 1.5 kgf/cm and more preferably at least 1.75kgf/cm in both the machine and transverse directions. For cells withdiameters greater than about 0.45 inch (11.4 mm), a tensile stress of atleast 2.0 kgf/cm is most preferable.

Another indicator of separator strength is its dielectric breakdownvoltage. Preferably the average dielectric breakdown voltage will be atleast 2000 volts, more preferably at least 2200 volts. For cylindricalcells with a diameter greater than about 0.45 in (11.4 mm), the averagedielectric breakdown voltage is most preferably at least 2400 volts. Ifthe dielectric breakdown voltage is too low, it is difficult to reliablyremove cells with defective or damaged separators by electrical testing(e.g., retention of a high voltage applied to the electrode assemblybefore the addition of electrolyte) during cell manufacturing. It isdesirable that the dielectric breakdown is as high as possible whilestill achieving other desirable separator properties.

The average effective pore size is another of the more importantindicators of separator strength. While large pores are desirable tomaximize ion transport through the separator, if the pores are too largethe separator will be susceptible to penetration and short circuitsbetween the electrodes. The preferred maximum effective pore size isfrom 0.08 μm to 0.40 μm, more preferably no greater than 0.20 μm.

The BET specific surface area is also related to pore size, as well asthe number of pores. In general, cell discharge performance tends to bebetter when the separator has a higher specific surface area, but theseparator strength tends to be lower. It is desirable for the BETspecific surface area to be no greater than 40 m²/g, but it is alsodesirable that it be at least 15 m²/g, more preferably at least 25 m²/g.

For good high rate and high power cell discharge performance a low areaspecific resistance is desirable. Thinner separators tend to have lowerresistances, but the separator should also be strong enough, limitinghow thin the separator can be. Preferably the area specific resistanceis no greater than 4.3 ohm-cm², more preferably no greater than 4.0ohm-cm², and most preferably no greater than 3.5 ohm-cm².

Separator membranes for use in lithium batteries are often made ofpolypropylene, polyethylene or ultrahigh molecular weight polyethylene,with polyethylene being preferred. The separator can be a single layerof biaxially oriented microporous membrane, or two or more layers can belaminated together to provide the desired tensile strengths inorthogonal directions. A single layer is preferred to minimize the cost.Suitable single layer biaxially oriented polyethylene microporousseparator is available from Tonen Chemical Corp., available from EXXONMobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separatorhas a 20 μm nominal thickness, and Setela 16MMS grade has a 16 μmnominal thickness.

The anode, cathode and separator strips are combined together in anelectrode assembly. The electrode assembly may be a spirally wounddesign, such as that shown in FIG. 1, made by winding alternating stripsof cathode, separator, anode and separator around a mandrel, which isextracted from the electrode assembly when winding is complete. At leastone layer of separator and/or at least one layer of electricallyinsulating film (e.g., polypropylene) is generally wrapped around theoutside of the electrode assembly. This serves a number of purposes: ithelps hold the assembly together and may be used to adjust the width ordiameter of the assembly to the desired dimension. The outermost end ofthe separator or other outer film layer may be held down with a piece ofadhesive tape or by heat sealing.

Rather than being spirally wound, the electrode assembly may be formedby folding the electrode and separator strips together. The strips maybe aligned along their lengths and then folded in an accordion fashion,or the anode and one electrode strip may be laid perpendicular to thecathode and another electrode strip and the electrodes alternatelyfolded one across the other (orthogonally oriented), in both casesforming a stack of alternating anode and cathode layers.

The electrode assembly is inserted into the housing container. In thecase of a spirally wound electrode assembly, whether in a cylindrical orprismatic container, the major surfaces of the electrodes areperpendicular to the side wall(s) of the container (in other words, thecentral core of the electrode assembly is parallel to a longitudinalaxis of the cell). Folded electrode assemblies are typically used inprismatic cells. In the case of an accordion-folded electrode assembly,the assembly is oriented so that the flat electrode surfaces at oppositeends of the stack of electrode layers are adjacent to opposite sides ofthe container. In these configurations the majority of the total area ofthe major surfaces of the anode is adjacent the majority of the totalarea of the major surfaces of the cathode through the separator, and theoutermost portions of the electrode major surfaces are adjacent to theside wall of the container. In this way, expansion of the electrodeassembly due to an increase in the combined thicknesses of the anode andcathode is constrained by the container side wall(s).

A nonaqueous electrolyte, containing water only in very small quantitiesas a contaminant (e.g., no more than about 500 parts per million byweight, depending on the electrolyte salt being used), is used in thebattery cell of the invention. Any nonaqueous electrolyte suitable foruse with lithium and active cathode material the may be used. Theelectrolyte contains one or more electrolyte salts dissolved in anorganic solvent. For a Li/FeS₂ cell examples of suitable salts includelithium bromide, lithium perchlorate, lithium hexafluorophosphate,potassium hexafluorophosphate, lithium hexafluoroarsenate, lithiumtrifluoromethanesulfonate and lithium iodide; and suitable organicsolvents include one or more of the following: dimethyl carbonate,diethyl carbonate, methylethyl carbonate, ethylene carbonate, propylenecarbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, methylformate, γ-butyrolactone, sulfolane, acetonitrile,3,5-dimethylisoxazole, n,n-dimethyl formamide and ethers. Thesalt/solvent combination will provide sufficient electrolytic andelectrical conductivity to meet the cell discharge requirements over thedesired temperature range. Ethers are often desirable because of theirgenerally low viscosity, good wetting capability, good low temperaturedischarge performance and good high rate discharge performance. This isparticularly true in Li/FeS₂ cells because the ethers are more stablethan with MnO₂ cathodes, so higher ether levels can be used. Suitableethers include, but are not limited to acyclic ethers such as1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl)ether,triglyme, tetraglyme and diethyl ether; and cyclic ethers such as1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran and3-methyl-2-oxazolidinone.

Specific anode, cathode and electrolyte compositions and amounts can beadjusted to provide the desired cell manufacturing, performance andstorage characteristics.

The cell can be closed and sealed using any suitable process. Suchprocesses may include, but are not limited to, crimping, redrawing,colleting and combinations thereof. For example, for the cell in FIG. 1,a bead is formed in the can after the electrodes and insulator cone areinserted, and the gasket and cover assembly (including the cell cover,contact spring and vent bushing) are placed in the open end of the can.The cell is supported at the bead while the gasket and cover assemblyare pushed downward against the bead. The diameter of the top of the canabove the bead is reduced with a segmented collet to hold the gasket andcover assembly in place in the cell. After electrolyte is dispensed intothe cell through the apertures in the vent bushing and cover, a ventball is inserted into the bushing to seal the aperture in the cellcover. A PTC device and a terminal cover are placed onto the cell overthe cell cover, and the top edge of the can is bent inward with acrimping die to hold retain the gasket, cover assembly, PTC device andterminal cover and complete the sealing of the open end of the can bythe gasket.

The above description is particularly relevant to cylindrical Li/FeS₂cells, such as FR6 and FR03 types, as defined in International StandardsIEC 60086-1 and IEC 60086-2, published by the InternationalElectrotechnical Commission, Geneva, Switzerland. However, the inventionmay also be adapted to other cell sizes and shapes and to cells withother electrode assembly, housing, seal and pressure relief ventdesigns.

Features invention and its advantages are further illustrated in thefollowing examples.

Example 1

FR6 type cylindrical Li/FeS₂ cells with spirally wound electrodeassemblies were made with varying electrode assembly void volumes percentimeter of interfacial electrode assembly height over a range ofabout 0.373 to about 0.455 cm³/cm. The void volumes were varied byadjusting the volume of the voids within the active material mixturecoated on the cathode. This was done with various combinations ofmixture formulations, thickness and packing. The separator material usedin all cells was a highly crystalline, unixially oriented, microporouspolypropylene material with a 25 μm nominal thickness.

Example 2

Samples of the cells from Example 1 were prepared for testing. For eachgroup with a given void volume per unit of height, some cells remainedundischarged and some cells were 50% discharged (discharged at a rate of200 mA for the time required to remove 50 percent of the ratedcapacity). Undischarged and 50% discharged cells were tested on anImpact Test, and the external temperature of each of the cells testedwas monitored during and for six hours after testing.

For the Impact Test a sample cell is placed on a flat surface, a 15.8 mmdiameter bar is placed across the center of the sample, and a 9.1 kgmass is dropped from a height of 61±2.5 cm onto the sample. The samplecell is impacted with its longitudinal axis parallel to the flat surfaceand perpendicular to the longitudinal axis of the 15.8 mm diameter barlying across the center of the cell. Each sample is subjected to only asingle impact.

None of the undischarged cells had an external temperature that exceeded170° C. The percentage of 50% discharged cells whose externaltemperatures exceeded 170° C. was plotted. The best curve fitting theplotted points is shown in FIG. 2, where the void volume per unit height(in cm³/cm) is on the x-axis, and the percentage of cells with anexternal temperature exceeding 170° C. is on the y-axis.

The Impact Test results show that as the electrode assembly void volumedecreases, the percentage of cells with an external temperatureexceeding 170° C. increases. From the graph in FIG. 2, 0% of the cellswith a void volume of approximately 0.45 cm³/cm of interfacial heightwould be predicted to have an external temperature exceeding 170° C.,and over 60% with a void volume of approximately 0.37 cm³/cm would bepredicted to exceed 170° C. The high external temperatures wereattributed to damage to the separator resulting in heat-generatinginternal short circuits.

Subsequent examination of both FR6Li/FeS₂ cells after different levelsof discharge revealed that a net increase in the FR6 cell totalelectrode volume, which becomes greater as discharge proceeds, causesbending and buckling of the electrode strips and collapsing of thecentral core of the electrode assembly by the time the cells are 50%discharged. In contrast, similar examination of Li/MnO₂ cells withspirally wound electrodes showed little if any discernable change in theelectrode assembly at 50% discharge. The difference between the activematerial volumes and the volumes of the discharge reaction productsprovides an explanation for the difference in the effects of dischargeon the spirally wound electrode assemblies of Li/FeS₂ vs. Li/MnO₂ cells.

Example 3

Four lots of FR6 cells were made, each with a separator made from adifferent material. A description of the separator materials is providedin Table 1, and typical separator properties, as determined by themethods described below, are summarized in Table 2. The separatormaterial used for Lot A is the same as that used in the cells inExample 1. Each cell contained about 1.60 g of electrolyte, theelectrolyte consisting of 9.14 weight percent LiI salt in a solventblend of 1,3-dioxolane, 1,2-dimethoxyethane and 3,5-dimethylisoxazole(63.05:27.63:0.18 by weight).

TABLE 1 Lot A Lot B Lot C Lot D highly highly amorphous amorphouscrystalline crystalline biaxially biaxially uniaxially uniaxiallyoriented oriented oriented oriented microporous microporous microporousmicroporous ultrahigh molecular polyethylene polypropylene polypropyleneweight polyethylene 20 μm thick 25 μm thick 20 μm thick 20 μm thick

TABLE 2 Property (units) Lot A Lot B Lot C Lot D Porosity (%) 38 38 4240 Max. effective pore 0.10 0.06 0.38 0.10 size (μm) Dielectricbreakdown 2700 2200 1600 2625 volt. (V) Tensile stress, TD 190 162 8441336 (kgf/cm²) Tensile stress, TD 0.475 0.324 1.688 2.672 (kgf/cm)Tensile stress, MD 1687 2671 1541 1828 (kgf/cm²) Tensile stress, MD4.218 5.342 3.082 3.656 (kgf/cm) Tensile elongation, 1000 790 440 320 TD(%) Tensile elongation, 120 54 260 225 MD (%) Area specific resist. 4.592.71 3.06 2.90 (Ω-cm²) BET spec. surf. area 44.0 48.9 16.2 36.4 (m²/g)

The same cell design was used for all of Lots A-D. The cell design wasone with greater amounts of active materials, a higher concentration ofFeS₂ in the cathode mixture and an increased electrode interfacialsurface area, as well as a lower anode:cathode total input capacityratio, than cells from Example 1 with an electrode assembly void volumeto interfacial height ratio of about 0.452, resulting in a 22 percentincrease in the cell interfacial capacity.

Example 4

Cells from each lot in Example 3 were discharged 50% and then tested onthe Impact Test. The percentage of cells exceeding 170° C. on the testwas 20% for Lot A, 80% for Lot B and 0% for Lots C and D.

By increasing the interfacial capacity 22 percent compared to cells fromExample 1 with an electrode assembly void volume to interfacial heightratio of about 0.452, the percentage of cells exceeding 170° C. on theImpact Test increased from 0% to 20%. Cells from Lot A had a reducedamount of void space to accommodate a net increase in volume ofdischarge reaction products compared the volume of the unreacted activematerials, increasing the adverse effects of discharge on the Li/FeS₂electrode assembly observed in Example 2.

The reduced separator material thickness in Lot B compared to Lot Acontributed in a further increase in the percentage of cells exceeding170° C. on the Impact Test from 20% to 80%.

Although the thicknesses of the separator materials in Lots C and D werethe same as the thickness of the Lot B separator, there were no cells ineither Lot C or Lot D. The results for Lots C and D were comparable tothose for cells from Example 1 with an electrode assembly void volume tointerfacial height ratio of about 0.452, even though the void volumewithin the cathode and the separator material thickness were bothreduced in Lots C and D.

Example 5

Three lots of FR6 cells were used to compare actual performance of FR6cells on relatively low rate and high rate discharge tests. The firstlot was Lot D from Example 3. Features of Lot D are summarized in Table3.

Cells in Lots E and F were made according to the prior art. The cells inLot F were like those in Example 1 with an electrode assembly voidvolume to interfacial height ratio of about 0.452. The features of LotsE and F are shown in Table 3. In Lot E the same separator material asthat in Lot F was used, but in Lot E the cathode mixture composition wasmodified and the cell interfacial capacity was increased by 18% comparedto Lot F. The use of a thinner (20 μm thick) separator in Lot D alloweda 22% increase in cell interfacial capacity compared to Lot F.

TABLE 3 Feature Lot D Lot E Lot F Anode Li—Al Li—Al Li—Al Li foilthickness (cm) 0.01524 0.01524 0.01524 Li foil width (cm) 3.899 3.8993.861 Li foil cut length (cm) 31.50 30.48 30.61 Li foil weight (g) 0.990.97 0.95 Li input capacity/cell 3859 3735 3664 (mAh) Anode interfacial3600 3485 3470 capacity/cell (mAh) Cathode Al current collector 0.002540.00254 0.00254 thickness (cm) Current collector 0.3313 0.3199 0.3186volume (cm³) Dry coating (wt %): FeS₂ 92.00 92.00 92.75  acetylene black1.40 1.40 2.5  graphite 4.00 MX15 4.0 MX15 2.25 KS6  binder 2.00 SEBS2.0 SEBS 2.00 PEPP  other 0.3 PTFE 0.3 PTFE 0.05 PEO  other 0.3 silica0.3 silica Coating real density 4.115 4.115 4.116 (g/cm³) Coatingthickness 0.0080 0.0080 0.0072 (ea. side) (cm) Coating loading (mg/cm²)21.26 21.26 16.98 Coating packing (%) 64 64 57 Coating width (cm) 4.0774.077 4.039 Cathode (coating) 29.85 28.83 28.96 length (cm) Coatingweight/cell (g) 5.17 5.00 3.97 Cathode input 4250 4110 3290capacity/cell (mAh) Cathode interfacial 4005 3877 3105 capacity/cell(mAh) Separator (2 pieces/cell) Material 20 μm PE 25 μm PP 25 μm PPLength/piece (cm) 39.5 39 39 Width/piece (cm) 44 44 44 Total volume(cm³) 0.431 0.425 0.532 Electrode Assembly Winding mandrel 0.4 0.4 0.4diameter (cm) Overwrap volume (cm³) 0.124 0.124 0.124 Interfacial height(cm) 3.899 3.899 3.861 Can Ni pltd. steel Ni pltd. steel Ni pltd. steelThickness (cm) 0.0241 0.0241 0.0241 Outside diameter (cm) 1.392 1.3921.379 Inside diameter (cm) 1.344 1.344 1.331 Cell Internal void volume(%) 10 10 12 Anode/cathode input 0.95 0.95 1.18 capacity Interfacialcapacity 3600 3485 3105 (mAh) Cathode cap./interfac. 724 701 578 vol.(mAh/cm³)

Example 6

Cells from each of Lots D, E and F were discharged at 200 mAcontinuously to 1.0 volt and at 1000 mA continuously to 1.0 volt. Table4 compares the results.

TABLE 4 Test Lot D Lot E Lot F  200 mA 3040 mAh 2890 mAh 2417 mAh 1000mA 2816 mAh 2170 mAh 2170 mAh

The following separator material properties are determined according tothe corresponding methods. Unless otherwise specified, all disclosedproperties are as determined at room temperature (20-25° C.).

-   -   Tensile stress was determined using an Instron Model 1123        Universal Tester according to ASTM D882-02. Samples were cut to        0.50 inches (1.27 cm) by 1.75 inches (4.45 cm). The initial jaw        separation was 1 inch (2.54 cm) and the strain rate was 2 inches        (5.08 cm) per minute. Tensile stress was calculated as applied        force divided by the initial cross sectional area (the width of        the sample perpendicular to the applied force times the        thickness of the sample).    -   Maximum effective pore diameter was measured on images made at        30,000 times magnification using a Scanning Electron Microscope        and covering an area of 4 μm×3 μm. For each separator sample, an        image was made of both major surfaces. On each image, the        largest pores were measured to determine the largest round        diameter that would fit within the pore wall (the maximum        effective diameter of the individual pores). The maximum        effective pore diameter of the sample was calculated by        averaging the maximum effective pore diameters of the two        largest pores on each side (i.e., the average of four individual        pores).    -   Porosity was determined by (1) cutting a sample of the        separator, (2) weighing the sample, (3) measuring the length,        width, and thickness of the sample, (3) calculating the density        from the weight and measurements, (4) dividing the calculated        density by the theoretical density of the separator polymer        resin, as provided by the separator manufacturer, (5)        multiplying the dividend by 100, and (5) subtracting this value        from 100.    -   Dielectric breakdown voltage was determined by placing a sample        of the separator between two stainless steel pins, each 2 cm in        diameter and having a flat circular tip, and applying an        increasing voltage across the pins using a Quadtech Model Sentry        20 hipot tester, and recording the displayed voltage (the        voltage at which current arcs through the sample).    -   Tensile elongation (elongation to break) was determined using an        Instron Model 1123 Universal Tester according to ASTM D882-02.        Samples were cut to 0.50 inches (1.27 cm) by 1.75 inches (4.45        cm). The initial jaw separation was 1 inch (2.54 cm) and the        strain rate was 2 inches (5.08 cm) per minute. Tensile        elongation was calculated by subtracting the initial sample        length from the sample length at break, dividing the remainder        by the initial sample length and multiplying the dividend by 100        percent.    -   Area Specific Resistance (ASR) was determined for separator        samples suspended in an electrolyte between two platinum        electrodes, using a Model 34 Conductance-Resistance Meter from        Yellow Springs Instrument, Yellow Springs, Ohio, USA, to make        resistance measurements. The electrolyte solution used was 9.14        weight percent LiI salt in a solvent blend of 1,3-dioxolane,        1,2-dimethoxyethane and 3,5-dimethylisoxazole (63.05:27.63:0.18        by weight). All testing was done in an atmosphere of less than 1        part per million of water and less than 100 parts per million of        oxygen. An electrically nonconductive sample holder, designed to        hold the separator sample with a 1.77 cm² area of the separator        exposed, was submerged in the electrolyte solution so that the        portion of the holder for holding the sample lay halfway between        two platinum electrodes, 0.259 cm apart. The resistance between        the electrodes was measured. The holder was removed from the        electrolyte, a separator sample inserted in the holder, and the        holder was slowly lowered into the electrolyte solution to the        same set level so that the sample was completely flooded with        electrolyte with no gas bubbles entrapped in the sample. The        resistance was measured. The ASR was calculated using the        formula:

ASR=A(R ₂ −R ₁ +ρL/A)

-   -   where A is the area of the exposed separator sample, R₂ is the        resistance value with the film present, R₁ is the resistance        value without the film, L is the separator sample thickness and        ρ is the conductivity of the electrolyte used.    -   Specific surface area was determined by the BET method, using a        TriStar gas adsorption analyzer from Micromeritics Instrument        Corporation, Norcross, Ga., USA. A sample of 0.1 g to 0.2 g of        the separator was cut into pieces of less than 1 cm² to fit the        sample holder, the sample was degassed under a stream of        nitrogen at 70° C. for 1 hour, and a pore size distribution        analysis was performed using nitrogen as the adsorbant gas and        collecting full adsorption/desorption isotherms.

It will be understood by those who practice the invention and thoseskilled in the art that various modifications and improvements may bemade to the invention without departing from the spirit of the discloseconcept. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

1. An electrochemical battery cell comprising a housing; a negativeelectrode strip consisting essentially of metallic lithium or lithiumalloyed with a metal; a positive electrode strip comprising irondisulfide; an electrolyte comprising at least one salt dissolved in anonaqueous electrolyte disposed within the housing; and a separatorcomprising a microporous membrane including polyethylene with athickness of less than 25 μm disposed between the negative and positiveelectrodes, wherein the cell has a ratio of a cathode interfacialcapacity to an electrode assembly interfacial volume of at least 710mAh/cm³ and a discharge capacity of at least 2950 mAh when discharged at200 mA continuously to a 1.0 volt cutoff.
 2. The cell according to claim1, wherein the thickness of the separator is 22 μm or less.
 3. The cellaccording to claim 1, wherein the thickness of the separator is 20 μm orless.
 4. The cell according to claim 1, wherein the thickness of theseparator is 16 μm or less.
 5. The cell according to claim 1, whereinthe electrolyte includes at least one solvent selected from: sulfolane,1,2-dimethoxyethane, 1,2-diehtoxyethane, di(methoxyethyl)ether,tri(glyme), tetragylme, diethyl ether, 1,3-dioxolane, tetrahydrofuranand 2-methyl tetrahydrofuran.
 6. The cell according to claim 5, whereinthe salt is at least one selected from: lithium iodide, lithiumperchlorate, lithium bromide and lithium trifluoromethanesulfonate. 7.The cell according to claim 1, wherein the separator has a tensilestress of at least 1.0 kgf/cm in both a machine direction and atransverse direction.
 8. The cell according to claim 7, wherein theseparator is biaxially oriented.
 9. The cell according to claim 1,wherein the polyethylene comprises ultrahigh molecular weightpolyethylene.